a) Field of the Invention
This invention relates to a system and method for the control of shifting of a vehicle automatic transmission, in which upon setting, for example, a normal speed range corresponding to a vehicle speed and an engine load, the control of the shifting is performed incorporating setting of the speed range by fuzzy inference on the basis of information on running state of a vehicle, information on driving behavior intended by a driver for the vehicle, information on conditions of a road on which the vehicle is running, and the like. In particular, this invention is concerned with a system and method for the control of shifting of a vehicle automatic transmission, which are designed to perform most suitable control of a downshift on a downhill road.
b) Description of the Related Art
A conventional vehicle automatic transmission suitable for use on an automotive vehicle is designed to perform a change-over in speed range such as an upshift or a downshift on the basis of a preset shift pattern upon receipt of information on an engine load such a throttle opening, information on a vehicle speed and information on a current speed range.
Such a conventional vehicle automatic transmission involves no particular serious problem in shifting the speed range on a flat road as in street running, whereby the shifting is smooth and gives no sense of incongruity. When running, for example, in a mountainous region, however, there are straight uphill roads and also frequently bent uphill roads as well as downhill slopes requiring a strong engine brake and gentle long downhill slopes. There are also drivers who abruptly accelerate their vehicles on a downhill slope and apply a strong brake immediately entering a corner.
While running in such a mountainous region, it is rather difficult to choose a speed range optimal to the state of running of the vehicle, the driving behavior intended by the driver, road conditions and the like. There is accordingly a demand for permitting good control of motion of a vehicle by a simple running operation even during running in a mountainous region so that better drive feeling and run feeling can be obtained.
For such a demand, it is known, for example, from Japanese Patent Application Laid-Open (Kokai) No. SHO 62-246546 or HEI 2-3738 to perform the so-called "fuzzy control" so that an optimal speed range corresponding to the above-described state of running of the vehicle can be selected.
These conventional shift control methods are designed to set optimal speed ranges by inferring all gear positions for running in both urban and mountainous regions in accordance with fuzzy inference. These conventional shift control methods making use of "fuzzy control" are however accompanied by the problem that they require many rules and hence a membership function of an intricate profile. This has led to the problems that a large-capacity computer is needed to put such a method into practical application and tuning of the control is difficult, thereby making it difficult to apply the control method to other types of automotive vehicles.
Further, additional incorporation of shift control by "fuzzy control" may result in the execution of shifting by such a small change in the running or driving state, e.g., running-over of a small bump or slight depression of an accelerator pedal that no shifting would take place according to the conventional art. This has led to the problem that the above shifting may give a sense of incongruity to drivers who are accustomed to running on flat roads such as street running under control by a conventional automatic transmission.
With a view to overcoming these problems, a shift control method was proposed in Japanese Patent Application Laid-Open (Kokai) No. HEI 2-212655. According to this shift control method, various parameters indicating the state of running of a vehicle are detected so that detection signals are produced. Based on the detection signals and a membership function set beforehand, fuzzy inference is conducted to determine the degree of running resistance. When this running resistance is greater than a predetermined value, a shift map for high-load running is selected in place of a shift map for normal running so that a speed range is determined by the shift map for high-load running.
According to this proposal, the same shift map is used for both straight uphill roads and frequently bent uphill roads, leading to the problem that carefully thought-out shift control can be hardly conducted to sufficient extents in the light of the above-mentioned various road conditions in mountainous regions and also intended driving behaviors. If the shift map for normal running is changed over to the shift map for high-load running based solely upon the occurrence of running resistance greater than the predetermined value and a speed range is then set in accordance with the latter map, the speed range so selected in accordance with the shift map for high-load running may become higher than the speed range which has been selected by the shift map for normal running. There is hence the potential problem that a sense of incongruity may be given to the driver.
A still further shift control method was hence proposed as disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-337157. According to this shift control method, an optimal speed range is selected based on a normal shift pattern set beforehand and in addition, an optimal speed range is also selected by fuzzy inference on the basis of at least one of information on the state of operation of a vehicle, information on driving behavior intended by a driver and information on road conditions. The optimal speed range selected based on the normal shift pattern is usually set as a speed range for an automatic transmission but only when the optimal speed range selected by the fuzzy inference is lower than the optimal speed range selected based on the normal shift pattern, the optimal speed range selected by the fuzzy inference is set as a speed range for the automatic transmission.
Among such shift control methods making use of "fuzzy control" as mentioned above, those designed to perform setting of a speed range by fuzzy inference instead of setting of a speed range according to shifting characteristics for normal running as needed--as disclosed, for example, in Japanese Patent Applications Laid-Open Nos. HEI 4-337157 and 2-212655--may select by fuzzy inference a speed range lower than a speed range--which would otherwise be selected according to shifting characteristics for normal running--for example, on a downhill road to obtain a necessary engine brake, and may then set it as a speed range for an automatic transmission.
In this case, the downshift operation for obtaining the engine brake may give a sense of incongruity to the driver.
According to shift control by shifting characteristics for normal running, a downshift or an upshift is performed, for example, based on vehicle speed information and engine load (throttle opening) information while using such a map as shown in FIG. 20. In this shift map, the solid line a is an upshift line from 3rd speed to 4th speed while the alternate long and short dash line b is a downshift line from 4th speed to 3rd speed.
If the driver operates a throttle valve (in other words, depresses an accelerator pedal) to point B when the vehicle speed and the throttle opening are as indicated by point A, a downshift operation is performed after passing across the downshift line b, that is, at point C. This downshift operation is generally accompanied by a shift shock, which becomes larger especially where importance is placed on its response characteristic. The driver however can expect the shift shock in this case so that he does not feel a sense of incongruity upon shifting.
Further, the vehicle speed may increase from point A to point D in some instances even when the throttle opening is kept unchanged (in other words, even when the driver does not change the amount of depression of the accelerator pedal). If the vehicle is running on a flat road at this time, the vehicle speed increases passing across the upshift line a as illustrated in the diagram so that an upshift operation is performed. If the vehicle is running on a downhill road, however, a downshift from 3rd speed to 2nd speed may be commanded instead to obtain an engine brake as a result of selection of a speed range by fuzzy inference. As a consequence, a downshift operation is performed.
The driver cannot foresee the downshift in the above case, so that a shift shock produced as a result of the downshift operation gives a surprise to the driver. The driver feels a sense of substantial incongruity at the time of the shifting especially in a shifting operation which places importance on the response characteristic and produces a large shock. This leads to the problem that the smoothness of a shifting operation which is an inherent characteristic feature to an automatic transmission will be lost.